1. Field of the Invention
The invention relates to a laser device capable of operating with enhanced quantum efficiency across a broadband of modulation frequencies. In particular, this may be achieved in the fixed impedance (50 xcexa9) microwave circuitry without the need for complex impedance matching. The invention also relates to a transistor-like device capable of delivering current gain across a broadband of frequencies, again without the need for complex impedance matching. The invention has advantages in the field of fibre optic communications and transmissions, and in RF applications due to its broadband nature and the ability to produce current gain.
2. Discussion of Prior Art
The operation of a conventional common base bipolar transistor is well know in the prior art, for example see W. Shockley, Bell Syst. Technology, J. 28, 435,1949. The crucial feature of this transistor is the transit of current from the low impedance input (base/emitter) circuit to a high impedance output (base/collector) circuit by means of minority carrier diffusion across a thin (base) semiconductor layer. The reverse biased collector output can therefore deliver current into a relatively high impedance load resistor.
It has been proposed to use a Beam Of Light Transistor (BOLT) [R. Rediker el al., Proc. IEEE 51, 218 1963] in which the transfer of current is achieved by converting the minority carriers into photons before transit across a xe2x80x9cbasexe2x80x9d layer in the device and then converting the photons back to a current at the output. However, the optical transfer cannot be achieved efficiently in such devices and current losses are always encountered and therefore current gain cannot be achieved.
In conventional microwave technology, to increase microwave power at high frequencies several individual transistors may be connected in parallel in order to achieve current gain. Such small devices have a high speed of operation, but the resistance of the parallel arrangement is small. In microwave circuitry, where the majority of microwave generation, transmission, reception and cable hardware is of 50 xcexa9 impedance, it is to difficult to match the low impedance of these devices to the 50 xcexa9 hardware over a broad frequency range. As conventional electronic transistors are three terminal devices, they may only be combined in parallel. This restricts the possibilities for thermal management.
Another problem associated with electrical transistors is that the displacement currents produced limit the speed of operation of the transistor. Furthermore, as the input circuit and the output circuit are coupled problems can arise due to feedback from the output circuit to the input circuit.
The transistor of the present invention overcomes the problems in known electronic and optical transistor devices. Furthermore, its characteristics and design make it suitable for application to fibre optic communications and the optical distribution of radiofrequency, microwave, mm-wave and digital signals in electronic systems such as phased array radars. In the field of fibre optic links, conventional semiconductor lasers are commonly used. The current technology for achieving directly modulated, broadband, high speed fibre optic links has been developed principally for digital communications systems. Typically, such fibre optic links operate at an upper frequency limit of around 10 GHz set by the response capability of the laser to the input signal. Conventional broadband fibre optic links, however, cannot deliver signal gain unless either electronic or optical amplifiers are included in the signal path. Furthermore, for use in fixed impedance environments, such as microwave circuits, a relatively narrow band impedance transformation is need to match the low impedance laser in order to minimise signal loss.
The invention overcomes the narrow bandwidth limitation associated with conventional lasers in that no impedance transformation is necessary. The laser may also be used in a transistor device to provide current gain across a broadband of modulation frequencies, from DC up to typically 30 GHz. For the purpose of this description, references to frequency shall be taken to mean modulation frequency rather than the optical emission frequency of the laser. Such a device therefore overcomes the impedance transformation problems encountered when conventional transistors are used in 50 xcexa9 microwave circuitry. The laser is also capable of providing both AC and DC signal gain in combination with a photodiode, which conventional lasers cannot deliver.
According to one aspect of the invention, a light emitting device, having an input impedance and a device quantum efficiency, for generating at least one beam of output radiation from an input current of electrons comprises;
at least two light emitting means, each having an impedance and an individual quantum efficiency, for converting the input current of electrons into a beam of output radiation,
characterised in that the light emitting means are electrically connected such that the device quantum efficiency is greater than or equal to the individual quantum efficiency of one of the light emitting means.
In one embodiment of the invention, the light emitting means are electrically connected in series such that the input impedance of the light emitting device is substantially equal to the sum of the impedances of the light emitting means and the device quantum efficiency may be substantially equal to the sum of the quantum efficiencies of the light emitting means.
In a preferred embodiment, the light emitting means may be electrically connected such that the input impedance of the light emitting device is substantially equal to 50 xcexa9 without additional circuitry or impedance matching elements.
In a preferred embodiment, the light emitting means may be electrically connected such that the input impedance of the light emitting device is substantially equal to 50 xcexa9 without additional circuitry or impedance matching elements.
The light emitting means, having a modulation frequency limit, wherein the input impedance of the light emitting device is substantially equal to 50 xcexa9 across a frequency range substantially from DC to the modulation frequency limit of each of the light emitting means.
The light emitting means may be p-n junctions, for example laser diodes or light emitting diodes. Typically, the laser diode devices maybe any one of AlGaAs, AlGaInP, AlGaInAs or AlGaInAsP laser diode devices. The p-n junctions may have an end face coated with a reflective coating.
According to another aspect of the invention, an optically coupled transistor for generating an output electrical signal comprises;
the light emitting device as herein described for emitting at least two beams of output radiation and
at least one photodetector for detecting the beams of radiation output from the light emitting device and for converting the beams of output radiation into an output electrical current.
In one embodiment of the optically coupled transistor, the one or more photodetectors may be a photodiode device.
For example, the optically coupled transistor may comprise at least two photodetectors, wherein the photodetectors are connected in any one of a series connection, a parallel connection or a series parallel connection.
The optically coupled transistor may also comprise one or more optical fibres for transmitting the beams of output radiation to the one or more photodetectors.
According to another aspect of the invention, a fibre optic link comprises one or more optical fibres having an input endface and an output endface, and also comprising the light emitting device as herein described, wherein the light emitting device is situated at the input endface of one or more optical fibres such that the beams of radiation output from the light emitting device are input to the one or more optical fibres.
According to another aspect of the invention, a method for distributing an input signal into a plurality of output channels comprises the steps of;
(i) outputting two or more beams of radiation from the light emitting device as herein described comprising at least two light emitting means and
(ii) inputting each of the two or more beams of output radiation into one of the output channels,
whereby the light emitting means are connected such that the device quantum efficiency is greater than or equal to the individual quantum efficiency of one of the light emitting means.
In a preferred embodiment, the method may comprise the step of inputting each of the two or more beams of output radiation into a different one of the output channels.
According to another aspect of the invention, an optical repeater for receiving an optical input signal and generating one or more optical output signals comprises;
a photodetector for receiving the optical input signal and converting the optical input signal into an electrical signal and
the laser device of any of claims for receiving the said electrical signal and outputting one or more optical signals.
In a preferred embodiment, the optical repeater also comprises amplification means for amplifying the electrical signal output from the photodetector.